French Onion Soup Grilled Cheese with Gruyère Crust Step-by-step instructions

Before you start

Before starting, have all your ingredients ready. Caramelize the onions in advance if possible, as they take about 20 minutes. Shred the cheeses and measure the sherry and beef stock.

Spread the garlic herb mayo on the bread slices and press them into Parmesan to create the crust. Preheat the skillet over medium heat so it's ready when you are.

Instructions

Caramelize and Prep

1. 1Caramelize the Onions
   In a large cast-iron skillet over medium heat, melt 1 tablespoon of butter. Add the sliced onions and cook, stirring occasionally, until deeply golden and caramelized, about 20 minutes. If they start to stick, add a splash of water. Once the onions are richly browned, add the dry sherry, beef stock, and fresh thyme leaves. Cook for 1 to 2 minutes, scraping up any browned bits, until the liquid is mostly absorbed. Remove from heat and set aside.
   Look for The onions are soft, dark brown, and glossy, and the sherry has mostly evaporated.
   Tip Patience is key — cook the onions slowly to bring out their natural sweetness.
2. 2Shred the Cheeses
   Shred the Comté and Gruyère cheeses if not already shredded. Combine them in a bowl. Spread the garlic herb mayonnaise evenly on one side of each sourdough slice. Sprinkle the grated Parmesan onto a plate and press the mayonnaise-coated side of each bread slice into the Parmesan, creating a crust.
   Look for The bread slices are evenly coated with mayonnaise and a thin layer of Parmesan.
   Tip Press firmly so the Parmesan adheres well — it will form a crispy crust when cooked.

The onions should be deeply browned, soft, and glossy, with the sherry and stock reduced to a glossy coating. The bread is coated with mayo and Parmesan, and the cheeses are shredded and ready. The kitchen smells sweet and savory from the onions.

Griddle and Melt

3. 3Assemble the Sandwiches
   Place two slices of bread, Parmesan-side down, on a clean work surface. Divide the caramelized onion mixture evenly between the two slices. Top with the shredded Comté and Gruyère mixture. Place the remaining bread slices on top, Parmesan-side up, to form two sandwiches.
   Look for The sandwiches are neatly stacked with the cheese and onions evenly distributed.
   Tip Don't overstuff — the cheese should melt and hold the sandwich together.
4. 4Griddle the Sandwiches
   Heat the same cast-iron skillet over medium heat and add the remaining butter. Once the butter is melted and sizzling, place the sandwiches in the skillet. Cook for 3 minutes per side, pressing down gently with a spatula, until the bread is golden brown and the cheese is melted. If the bread browns too quickly, reduce the heat slightly.
   Look for The bread is golden brown and crisp, and the cheese is visibly melted and oozing.
   Tip Use a spatula to press the sandwiches gently — this helps the cheese melt evenly and creates a crisp crust.

The sandwiches are in the skillet, sizzling gently. The bread is turning golden brown, and the cheese is beginning to melt. The air is filled with the aroma of toasted bread and melted cheese. The crust is forming a crisp, golden edge.

Rest and Serve

5. 5Rest and Slice
   Remove the sandwiches from the skillet and let them rest for 2 minutes on a cutting board. This allows the cheese to set slightly and prevents it from running out when slicing. Use a sharp knife to cut each sandwich in half, revealing the gooey, stretched cheese pull.
   Look for The sandwiches are crisp and golden, and the cheese is molten but not running.
   Tip A brief rest makes slicing cleaner and the cheese pull more dramatic.

The sandwiches are out of the skillet, resting on a cutting board. The crust is deeply golden and crisp, and the cheese is molten and stretchy. Cutting into the sandwich reveals a long, gooey cheese pull. The flavors are rich, savory, and deeply satisfying.

Common mistakes

One common mistake is using too high heat, which burns the bread before the cheese melts. Keep the heat at medium and press the sandwiches gently. Another mistake is overloading the sandwich with too many onions, causing it to fall apart.

Stick to the amounts specified. Also, be sure to let the sandwiches rest for a couple of minutes before slicing to allow the cheese to set slightly.

How to tell it’s done

The sandwiches are done when the bread is deeply golden brown and crisp on both sides, and the cheese is fully melted and oozing out slightly.

To check for doneness, carefully lift a corner of the sandwich with a spatula — the bottom should be golden and the cheese should be bubbly.

The internal temperature of the cheese should reach at least 160°F (71°C) to ensure it's fully melted and safe to eat.